The basis of these experiments, which are designed to throw some light on the question of calcification, is the observation of Shipley made a year ago that the epiphyseal cartilage of rachitic rats when placed in the serum of rachitic rats does not calcify, but does calcify when placed in the serum of nonrachitic rats. Our experiments have been conducted in a number of different ways. To begin with, there are certain precautions that should be observed. The rats should be completely rachitic, that is, there must be no calcification whatever in their cartilages; they must be fed continuously up to the time that they are killed; they must be killed without anesthesia and the slabs of tissues, cut from the extremity of the bones, must not be too thin. Sterile precautions should be observed in the preparation and the handling of the solutions and of the cartilage.